Since olefin polymers, in particular polypropylene (hereinafter referred to as PP, occasionally), are crystalline macromolecular compounds, the olefin polymers exhibit excellent rigidity, tensile strength, heat resistance, chemical resistance, optical properties and workability and have low specific gravities. Therefore, the olefin polymers are widely used in various fields such as injection molded articles, containers and packaging materials.
As the catalyst system for polymerizing α-olefins, many catalysts comprising a solid catalyst component, an organoaluminum compound and, where necessary, an electron-donating compound have been disclosed. The solid catalyst component comprises magnesium, titanium, halogen elements and, where necessary, an electron-donating compound. When an α-olefin polymer is produced using such a catalyst, in general, hydrogen is used as the chain transfer agent. However, this process has a drawback in that hydrogen must be added in a great amount in order to obtain an α-olefin polymer exhibiting high fluidity and, as the result, the stereoregularity deteriorates or productivity decreases due to a decrease in the monomer concentration in the polymerization field.
As described above, it is the general practice that the amount of hydrogen as the chain transfer agent during the polymerization is increased to improve fluidity of the polymer from the standpoint of workability. However, since the resistance of a reactor to pressure is limited, the concentration of the monomer which can be placed into the reactor decreases due to the increase in the concentration of hydrogen. This causes a decrease in the efficiency of the catalyst and an economic disadvantage arises.
Thus, the present invention has a first object of providing a process for industrially advantageously producing an α-olefin polymer having extremely high stereoregularity, exhibiting excellent fluidity and containing a decreased amount of catalyst residues in the polymer.
A block copolymer of propylene (hereinafter referred to as block PP, occasionally) is produced, in general, in accordance with a process comprising producing a homopolymer of propylene by polymerization of propylene, followed by producing a copolymer by copolymerization with other momoners. The properties required for block PP are mechanical properties derived from the homopolypropylene and impact resistance in an excellent balance. It is also required that the molding property, appearance and elongation are excellent. It is known that such requirements can be satisfied by a structure having a homopolymer portion exhibiting excellent fluidity and a copolymer portion having a relatively high molecular weight.
To enhance fluidity of the homopolymer portion, in general, the amount of hydrogen used as the chain transfer agent is increased during the polymerization. However, when hydrogen used in the homopolymerization in the first stage affects the condition of the copolymerization in the second stage, in other words, when hydrogen is not removed or is only partially removed between the first stage and the second stage, an increased amount of hydrogen in the stage of the homopolymerization causes a decrease in the molecular weight of the copolymer portion due to an increase in the amount of hydrogen in the reactor used in the copolymerization of the second stage and block PP having the desired impact resistance cannot be obtained. To improve the impact resistance of block PP, it is necessary that hydrogen be removed completely between the first stage and the second stage and the additional facilities be installed for this purpose. Moreover, the inner pressure of polymerization apparatuses increases when the amount of hydrogen is increased during the polymerization. Since the resistance of the polymerization apparatuses to pressure is limited, the amount of hydrogen used during the polymerization is naturally limited. In this case, fluidity of the obtained homopolypropylene is limited and block PP having the desired properties cannot be obtained.
When fluidity of the homopolymer portion in block PP is increased by decomposition of the polymer, the fluidity expected from the melt index (MI) cannot be obtained. Moreover, a problem arises in that physical properties such as impact resistance deteriorate since the molecular weight of the block portion also decreases.
Therefore, development of a process for producing a homopolymer having a high fluidity without a decrease in the molecular weight of the copolymer portion or an increase in the inner pressure of the reactor has been desired.
The present invention has a second object of providing a process for efficiently producing a block polypropylene comprising a homopolymer portion exhibiting a high fluidity and a copolymer portion having a high molecular weight.